Blood treatment apparatuses with a blood treatment unit through which a patient's blood flows are generally known. These include for example the familiar hemodialysis, hemofiltration or hemodiafiltration apparatuses. These known blood treatment apparatuses can be operated in a single-needle mode or a two-needle mode.
The blood treatment apparatuses for both single-needle and two-needle operation have an extracorporeal blood circuit with an arterial blood conduit that leads to the blood treatment unit, and a venous blood conduit that leads away from the blood treatment unit.
In the case of the two-needle technique, the blood is taken from one of the patient's blood vessels via an arterial needle; it is then directed into the blood treatment unit of the blood treatment apparatus, and is returned via a venous needle into one of the patient's blood vessels. Replaceable hose systems with a blood supply line and a blood return line, to which the two needles are connected, are used for taking and returning the blood. These single-use hose systems are also referred to as “disposables”.
In the case of the single-needle technique, the blood is extracted and returned via a single needle, to which both the arterial and the venous blood conduits are connected. The blood that is taken from the patient is stored in a reservoir during an arterial phase, in order to be led out of the reservoir into the patient's blood circuit in a venous phase, through the same needle.
A blood treatment apparatus for single-needle operation is known from EP-A-0 472 480 B1. In the case of one embodiment of this known blood treatment device, two blood expansion chambers are provided for the temporary storage of blood, which are arranged upstream and downstream of the blood treatment unit. The blood treatment unit has a control device that keeps the pressure in the expansion chambers essentially constant. Level sensors are provided, to detect the fluid level in the expansion chambers.
The document DE 10 2005 001 779 A1 describes a set for a disposable, for operating a blood treatment apparatus in a single-needle or a two-needle mode. Besides the blood supply and blood return conduit for connection to a blood treatment unit, the disposable includes an expansion unit, which for single-needle operation can be coupled to the air separation unit in order to increase the volume. During single-needle operation, in the arterial phase blood is transported through the blood supply conduit into the blood treatment unit and out of the blood treatment unit into the air separation and expansion unit, wherein the blood supply to the patient is interrupted. In the course of this, a given pressure is built up in the air separation and expansion unit, this pressure being monitored with a pressure measurement unit. With a compressed air unit, by operating an air pump that is connected between a tank and the expansion unit, a given pressure can be set in the expansion and air separation unit. It is also proposed that the blood volume in the expansion and air separation unit be calculated with the aid of the measured values from three pressure sensors and the known system volumes. It is furthermore proposed that the air pump be used to control the pressure during the venous phase, so that the transportation rate of the blood can be optimally adapted.
For the proper operation of blood treatment apparatuses in both single-needle and two-needle operation, it is important that blood is supplied back to the patient at a predetermined volume flow rate.